Description

The charging technique is simple CV and CC, and the LiFePO4 batteries are treated as if they were capacitors. The power source may be TEG (like the powerpot), solar panels, or a dynamo. It can also be charged quickly directly from a car battery. The supply current for the wattmeter is with :
NOKIA 5110 : 7.5 mA.
JLX12864G-086 : 15mA

The device is not (yet) a MPPT charger, even though the wattmeter circuit provides a good basis for taking that step in the future.

Extra feature, “or-ing” diodes:
By connecting several diodes (as many as you wish) in parallel to the primary blocking diode, it is possible to charge multiple batteries at once. However, the most discharged battery should be placed directly after the primary diode.

Details

Background:

I wanted a charger for portable power that could be used with
solar panels, TEG
elements and dynamo. It was also important that the charger could take all
available power. I quickly realized that LiFePO4 are the
best batteries available. LiFePO4 batteries have a long
shelf-life, can tolerate > 2000 charge cycles, and can accept high
charge/discharge currents. They have very little internal resistance,
not far from that of a super-capacitor. One of the toughest brands is
the A123
Nanophosphate, with 7000 cycles at 1C before it is reduced to
80% of capacity (it can be used for spot welding or to start
a car). I also wanted a wattmeter that could work down to 2.8 V
(discharged LiFePO4). Long life and durability had highest
priority.

Usage:

Adjust CV (Vout) on the
step-down to battery float voltage

Adjust CC (current limit) for
smaller batteries (or use 5 A to protect the charger)

The heart of the charger is the step-down
converter from prodctodc. I have tried at least 10 different
models and what I like about this one is it does not heat up much,
has a high efficiency, and can take a lot of beating. The
modifications I have made to this circuit are simple; I have just
inserted a reverse blocking Schottky
diodeand then moved the voltage sense resistor to the new +Vout.
I also replaced the 470 µF Sanyo capacitors with
equivalents from Rubycon. This step is not necessary; it is only
done as a precaution to ensure a long life. The original Sanyo
capacitor has low ESR and seems to work fine. Finally, I inserted a
0.02
Ω current sense resistor before −Vin. This
resistor is used by the wattmeter. By connecting several diodes (as
many as you wish) in parallel to the primary blocking diode, it is
possible to charge multiple batteries at once. However, the most
discharged battery should be placed after the primary diode (with CV
sense). This is very useful when you leave your basecamp for a while
and want to charge several cells. This is also a poor man's
balancing circuit; that is, the batteries will soon have equal
charge. Personally, I prefer four single cell LiFePO4
that can be combined into one 12 V battery, instead of one hard–wired 12 V
battery.

Charging a battery is similar to a short-circuit, where the CC is
the current limit. When the power supply is too weak, Vin
will break down to approximately 1 V above Vout (battery
voltage). To achieve a kind of poor man's MPPT, one should select
an appropriate open voltage for the power supply (solar panel =
1.25xVin). A better solution would be MPPT (a future
improvement). However, it is still possible to catch 90% of available
energy from a 5.5 V panel using this simple approach.

Wattmeter design:

The heart of the wattmeter is a Attiny861 which has differential gain x1, x8, x20, x32. This means there is no need for an op-amp. Attiny861 also has four different Voltage reference sections ( 1.1V, 2.56V, Vcc (3.3V),Aref ). This gives a lot of options. I prefer to use the internal 1.1V and 2.56V Vref ( since this makes the circuit less dependent on the 3.3V supply). The 3.3V (buck-boost) voltage supply to the wattmeter is a gutted ebay voltmeter. It also has a voltage divider. The wattmeter is supplied from the battery ( Vout ). The buck-boost circuit ensure 3.3V when battery voltage varies between 1.7-25V. And finally a low power...

The one I work with has a feature of sometimes forgetting "screen orientation", my workaround is repeating this command before writing any text. My guess it's a light sensitive part at the bottom of the display that triggers the effect. Once I've covered it with tape..problem seems to be gone.

In a solar cell, the parameter most affected by an increase in
temperature is the open-circuit voltage -2.2 mV/°C

The short-circuit current, Isc, increases slightly with temperature, since the band gap energy, EG,
decreases and more photons have enough energy to create e-h pairs.
However, this is a small effect and the temperature dependence of the
short-circuit current from a silicon solar cell is

I have compared the 6.4 dm² panels with my 11.2 dm² under harsh conditions. The light intensity peaked at 4300 lux and it was impossible to distinguish the contour of the sun. I managed to charge close to 0.5Ah. I think at least 22 dm² ( 30W panel ) is necessary to do something meaningful in this kind of light. ( When it's snowing I've measured 170 lux )

A cloudy winter day, but I could see the contour of the sun behind the clouds. I measured around 12000 lux peak. For this test a geared up and tested two of my "new" 7.5W panlels. You can read more about them at http://cottonpickersplace.com/ . I think these panels are very good and they come in many sizes. I prefer panels that are waterproof and has exposed soldering points. I'm evaluating if the slightly smaller panels (1.6x2dm) or this size (2.8x2 dm) is the best.

With just 6.4 dm² panel area in december (winter in Sweden) I could charge a 2Ah LiFePO4 in less than three hours. The light sensor is just a rough estimate ( It's still uncalibrated ). It is however worth to note that with a drop in temperature the efficiency of the panel increases. As an example a cell that produces 0.6A at 25 °C will produce 0.68A at 0 °C.

They are similar to the A123s, cant get quite as high in terms of charge and discharge but they are a lot cheaper and easy to configure in packs with simple screws, washers and plates along with battery holders.

Hey, do you have any reading material on lifepo4 batteries and how to charge them? I'm planning a bicycle computer project and figured those batteries would be great to use at german wheather conditions (can get goo hot or cold for lipos). Anyways great project!

My approach is to avoid over/under voltage and stay between 2.9 - 3.55V
Most of the energy in a LiFePO4 is 3-3.4V. Charging with < 1-2C is probably more efficient than fast charging ( check the spec for your battery ). In short I treat the LiFePO4 battery like a capacitor. In my case I don't have a power source strong enough to stress it (except car battery )...my charger will break before. My reason for CC is to limit the stress on the charger,. Another reason is when I use it to charge other batteries like Li-ion ( 2s1p camera batteries ).